The present invention relates to an electric motor, a stator of the electric motor, and a manufacturing method for the stator. More specifically, the present invention relates to an electric motor having a structure in which teeth made of a powder magnetic core material are fitted to a yoke, a stator of such an electric motor, and a manufacturing method for the stator.
FIG. 1 is a top plan view illustrating a prior art electric motor 10. The electric motor 10 has a stator 11 and a rotor 18. The stator 11 includes a plurality of teeth 12, a yoke 14, and a plurality of coils 16. The teeth 12 and the yoke 14 are molded separately. The stator 11 is formed in the following manner. First, the coils 16 are wound about the teeth 12, and then, an end 12b of each tooth 12 is fitted in a fitting groove formed in the yoke 14.
The teeth 12 are parts of the stator 11 that function as field poles. Typically, the teeth 12 are formed of steel. However, since the specific resistance of steel is 10−6 Ωm or less, the eddy-current loss of the teeth 12 is relatively great. Japanese Laid-Open Patent Publication Nos. 10-225038, 2004-289899, 2003-199319, 2002-165410, 2000-184634, and 11-275781 each disclose a tooth 12 shown in FIG. 2, which is formed by laminating copper plates 12a. Since the specific resistance of the tooth 12 along a direction of lamination of the steel plates 12a is greater than that of a tooth formed by a single steel member, the eddy-current loss is reduced.
Materials for forming the tooth 12 include powder magnetic core material. The powder magnetic core material refers to a material formed by mixing powder of metal magnetic material such as iron with resin. The tooth 12 is formed by pressure molding a powder magnetic core material. The specific resistance of the tooth 12, which is formed of a powder magnetic core material, is 10−4 Ωm or more. Thus, the eddy-current loss of the tooth 12 is reduced.
Referring to FIG. 3, Japanese Laid-Open Patent Publication No. 2004-197157 discloses a tooth 12 formed of a powder magnetic core material. The tooth 12 of FIG. 3 does not have a laminated structure, but is molded as an integral body. The tooth 12 of FIG. 3 is therefore advantageous in reducing the number of components.
Typically, the yoke 14 is formed of steel. Specifically, the yoke 14 is formed by laminating steel plates. Alternatively, the yoke 14 may be formed of a powder magnetic core material to reduce the eddy-current loss.
A tooth 12 made of a powder magnetic core material is easily damaged when fitted to the yoke 14.
When the tooth 12 made of a powder magnetic core material is fitted to the yoke 14 made of laminated steel plates, a part in the end 12b of the tooth 12 that contacts a wall surface defining the groove of the yoke 14 is likely to be damaged. Also, when the tooth 12 is fitted to the yoke 14, the end 12b of the tooth 12 is likely to be broken.
In the case where the tooth 12 and the yoke 14 are both made of a powder magnetic core material, fitting the tooth 12 to the yoke 14 is likely to damage not only the tooth 12, but also the wall surface defining the groove.